Buggy code:
public class Logic { private static final String MESSAGE_WELCOME = "Welcome!<seq2seq4repair_space>%1$s<seq2seq4repair_space>is<seq2seq4repair_space>ready<seq2seq4repair_space>for<seq2seq4repair_space>use" ; private static final String MESSAGE_DELETE_FAIL = "failed<seq2seq4repair_space>to<seq2seq4repair_space>delete" ; private static final String MESSAGE_DELETE_SUCCESS = "deleted<seq2seq4repair_space>task<seq2seq4repair_space>%1$s" ; private static final String MESSAGE_CLEAR_FAIL = "failed<seq2seq4repair_space>to<seq2seq4repair_space>clear<seq2seq4repair_space>all<seq2seq4repair_space>tasks" ; private static final String MESSAGE_CLEAR_SUCCESS = "cleared<seq2seq4repair_space>all<seq2seq4repair_space>tasks" ; private static final String MESSAGE_ADD_SUCCESS = "added<seq2seq4repair_space>\"%1$s\"" ; private static final String MESSAGE_ADD_FAIL = "failed<seq2seq4repair_space>to<seq2seq4repair_space>add<seq2seq4repair_space>task" ; private static final String MESSAGE_EDIT_SUCCESS = "edited<seq2seq4repair_space>task<seq2seq4repair_space>%1$d" ; private static final String MESSAGE_EDIT_FAIL = "failed<seq2seq4repair_space>to<seq2seq4repair_space>edit<seq2seq4repair_space>task" ; private static final String MESSAGE_DISPLAY_ALL = "displaying<seq2seq4repair_space>all<seq2seq4repair_space>tasks" ; private static final String MESSAGE_REDO_ERROR = "redo<seq2seq4repair_space>error" ; private static final String MESSAGE_REDO_NOTHING = "nothing<seq2seq4repair_space>to<seq2seq4repair_space>redo" ; private static final String MESSAGE_REDO_SUCCESS = "redo<seq2seq4repair_space>successful" ; private static final String MESSAGE_UNDO_ERROR = "undo<seq2seq4repair_space>error" ; private static final String MESSAGE_UNDO_NOTHING = "nothing<seq2seq4repair_space>to<seq2seq4repair_space>undo" ; private static final String MESSAGE_UNDO_SUCCESS = "undo<seq2seq4repair_space>successful" ; private static final String MESSAGE_INVALID_COMMAND = "invalid<seq2seq4repair_space>command:<seq2seq4repair_space>\"%1$s\"" ; private String fileName ; private Parser parser ; private Storage storage ; private History history ; public Logic ( String fileName ) { } public UserInterfaceContent executeUserInput ( String userInput ) { } public UserInterfaceContent initialLaunch ( ) { } private UserInterfaceContent determineActions ( Command command , String userInput ) { } private UserInterfaceContent executeAdd ( Command command ) { } private UserInterfaceContent executeClear ( Command command ) { } private UserInterfaceContent executeDelete ( Command command ) { } private UserInterfaceContent executeDisplay ( ) { } private UserInterfaceContent executeEdit ( Command command ) { } private UserInterfaceContent executeRedo ( ) { State newState ; <START_BUG> if ( ( ( history . getCurrentState ( ) ) == null ) || ( ( newState = history . goForward ( ) ) == null ) ) { <END_BUG> return new UserInterfaceContent ( Logic . MESSAGE_REDO_NOTHING , storage . getAllTasks ( ) ) ; } State redoState ; Command doCommand = newState . getDoCommand ( ) ; if ( ( doCommand . getType ( ) ) == ( CommandType . ADD ) ) { redoState = storage . addTask ( doCommand ) ; } else if ( ( doCommand . getType ( ) ) == ( CommandType . CLEAR ) ) { redoState = storage . clearAll ( ) ; } else if ( ( doCommand . getType ( ) ) == ( CommandType . DELETE ) ) { redoState = storage . deleteTask ( doCommand ) ; } else if ( ( doCommand . getType ( ) ) == ( CommandType . EDIT ) ) { redoState = storage . editTask ( doCommand ) ; } else { redoState = new State ( null , null , null , false ) ; } if ( redoState . isSuccessful ( ) ) { return new UserInterfaceContent ( Logic . MESSAGE_REDO_SUCCESS , storage . getAllTasks ( ) ) ; } history . goBack ( ) ; return new UserInterfaceContent ( Logic . MESSAGE_REDO_ERROR , storage . getAllTasks ( ) ) ; } private UserInterfaceContent executeSearch ( Command command ) { } private UserInterfaceContent executeUndo ( ) { } private String formatAddSuccess ( String text ) { } private String formatDeleteSuccess ( int deleteLineNumber ) { } private String formatEditSuccess ( int editLineNumber ) { } private String formatInvalidCommand ( String userInput ) { } private String formatWelcomeMessage ( ) { } private UserInterfaceContent invalidCommand ( String message ) { } private UserInterfaceContent unknownCommand ( String userInput ) { } }

Fix:
if ( ( newState = history . goForward ( ) ) == null ) {